Potted header for hollow fiber membranes

ABSTRACT

A filtration device is provided for withdrawing permeate essentially continuously from a multicomponent aqueous substrate containing growing microorganisms in a reservoir. A vertical skein of fiber is scrubbed with coarse bubbles which emanate from a conversion baffle positioned under the skein. The substrate is aerated with fine bubbles in a size range small enough to transfer oxygen to the substrate efficiently. The baffle traps the fine bubbles and converts them to coarse bubbles which are effective to scrub the fibers. In the most preferred embodiment, the finished headers of the skein are derived from composite headers comprising a fixing lamina of resin in which the fibers are potted near their terminal ends, and a fugitive lamina of fugitive powdery material in which the terminal ends of the fibers are potted. The fugitive lamina is removed, preferably by dissolving the powder, e.g. finely divided common salt in water.

This is a division of patent application Ser. No. 09/258,999 filed onFeb. 26, 1999, now U.S. Pat. No. 6,042,677 which in turn is a divisionof patent application Ser. No. 08/896,517 filed Jun. 16, 1997, issued asU.S. Pat. No. 5,910,250.

Which is a continuation-in-part application of Ser. No. 08/514,119 filedAug. 11, 1995, to be issued as U.S. Pat. No. 5,639,373, and of Ser. No.08/690,045 filed Jul. 31, 1996 now U.S. Pat. No. 5,783,083.

BACKGROUND OF THE INVENTION

This invention relates to the utilization of bubbles of air in two sizeranges, one fine, the other coarse (defined below), to accomplishdifferent functions in the operation of a vertical skein of hollow fibermembranes for filtration of an aqueous medium from a biomass; and to amethod of forming a header for potting the ends of the fibers in thevertical skein so as to collect permeate.

The term “vertical skein” in the title (hereafter “skein” for brevity),specifically refers to an integrated combination of structural elementsincluding (i) a multiplicity of vertical fibers of substantially equallength; (ii) a pair of headers in each of which are potted the opposedterminal portions of the fibers so as to leave their ends open; and,(iii) permeate collection means held peripherally in fluid-tightengagement with each header so as to collect permeate from the ends ofthe fibers.

The term “fibers” is used for brevity, to refer to “hollow fibermembranes” of porous or semipermeable material in the form of acapillary tube or hollow fiber. The term “substrate” refers to amulticomponent liquid feed. A “multicomponent liquid feed” in this artrefers, for example, to fruit juices to be clarified or concentrated;wastewater or water containing particulate matter; proteinaceous liquiddairy products such as cheese whey, and the like. The term “particulatematter” is used to refer to micron-sized (from 1 to about 44 μm) andsub-micron sized (from about 0.1 μm to 1 μm) filtrable matter whichincludes not only particulate inorganic matter, but also dead and livebiologically active micro-organisms, colloidal dispersions, solutions oflarge organic molecules such as fulvic acid and humic acid, and oilemulsions.

The term “header” is used to specify a solid body in which one of theterminal end portions of each one of a multiplicity of fibers in theskein, is sealingly secured to preclude substrate from contaminating thepermeate in the lumens of the fibers. Typically, a header is acontinuous, generally rectangular parallelpiped (prism), or, acylindrical disk of arbitrary dimensions formed from a natural orsynthetic resinous material.

Except for their opposed ends being potted, there is no physicalrestraint on the fibers of a skein. The term “skein fibers” is usedherein to refer to plural arrays. An “array” refers to plural,essentially vertical fibers of substantially equal lengths, the one endsof each of which fibers are closely spaced-apart, either linearly in thetransverse (y-axis herein) direction to provide at least one row, andtypically plural rows of equidistantly spaced apart fibers. Lesspreferably, a multiplicity of fibers may be spaced in a random pattern.Typically, plural arrays are potted in a header and enter its face in agenerally horizontal x-y plane (see FIG. 3). The width of a rectangularparallelpiped header is measured along the x-axis, and is the relativelyshorter dimension of the rectangular upper surface of the header; and,the header's length, which is its relatively longer dimension, ismeasured along the y-axis.

The parent patent and application teach how to use bubbles to maintainclean fiber surfaces during microfiltration (MF) or ultrafiltration (UF)with a skein. The problem was that coarse bubbles which are mosteffective for scrubbing fibers are inefficient and uneconomical toprovide an oxygen-containing gas (typically air, or air enriched withoxygen, occasionally pure oxygen) required for growth of microorganismsin the aqueous substrate. When furnished as fine bubbles, oxygen isdissolved in the substrate with an efficiency up to 10 times as much aswhen it is furnished as coarse bubbles. Particularly in large systemsfor the microfiltration of liquids, the combination of a foraminousconversion baffle and means to channel fine bubbles under the conversionbaffle above which the skein is disposed. provides a surprisinglyeffective and economical solution.

In a vertical skein, all fibers in the plural rows of fibers, risegenerally vertically while fixedly held near their opposed terminalportions in a pair of opposed, substantially identical headers to formthe skein of substantially parallel, vertical fibers. Typically theopposed ends of a multiplicity of fibers are potted inclosely-spaced-apart profusion and bound by potting resin, assuring afluid-tight circumferential seal around each fiber in the header andpresenting a peripheral boundary around the outermost peripheries of theoutermost fibers. The position of one fiber relative to another in askein is not critical, so long as all fibers are substantiallycodirectional through one face of each header, open ends of the fibersemerge from the opposed other face of each header, and substantially noterminal end portions of fibers are in fiber-to-fiber contact. Thevertical skein is not the subject matter of this invention and any priorart vertical skein may be used. Further details relating to theconstruction and deployment of a most preferred skein are found in theparent U.S. Pat. No. 5.639,373, and in Ser. No. 08/690,045, the relevantdisclosures of each of which are included by reference thereto as iffully set forth herein.

For MF at about ambient pressure, a skein securing at least 10,preferably from 50 to 50,000 fibers, each generally at least 0.5 m long,is disposed within a reservoir of substrate, above the conversionbaffle.

The fibers divide a reservoir into a “feed zone” and a withdrawal zonereferred to as a “permeate zone”. The feed (substrate) is introducedexternally (referred to as “outside-in” flow) of the fibers, andresolved into “permeate” and “concentrate” streams. The skein, or a bankof skeins is most preferably used for either MF or UF with “outside-in”flow.

The unique method of forming a header disclosed herein allows one toposition a large number of fibers, in closely-spaced apart relationship,randomly relative to one another, or, in a chosen geometric pattern,within each header. It is preferred to position the fibers in arraysbefore they are potted to ensure that the fibers are spaced apart fromeach other precisely, and, to avoid wasting space on the face of aheader; it is essential, for greatest reliability, that the fibers notbe contiguous. By sequentially potting the terminal portions of fibersin stages as described herein, the fibers may be cut to length in anarray, either after, or prior to being potted. The use of a razor-sharpknife, or scissors, or other cutting means to do so, does not decreasethe open cross-sectional area of the fibers' bores (“lumens”). The solidpotting resin forms a circumferential seal around the exterior terminalportions of each of the fibers, open ends of which protrude through thepermeate-discharging face of each header, referred to as the “aft” face.

The system disclosed herein is most preferably used to oxygenate mixedliquor in activated sludge, such as is generated in the bioremediationof wastewater. An oxygen-containing gas serves the dual purpose ofproviding fine bubbles for living microbes as well as coarse bubbles forscrubbing the fibers. Surface portions of the fibers are contacted bysuccessive bubbles as they rise, whether the air is suppliedcontinuously or intermittently, and the fibers are kept “awash inbubbles.”

SUMMARY OF THE INVENTION

It has been discovered that very small or “fine” bubbles in a size rangefrom adapted to provide a desirable utilization rate by microorganismsin a substrate, preferably from 0.5 mm to 5 mm, measured within afine-bubble discharging zone about 10 cm from the gas-dischargingsurface of a gas-distribution means, may be used to furnish the gasrequirements of a live and growing biomass, and then trapped as a massof gas in a trapping zone between the biomass and a vertical skeindisposed directly above the zone. The trapping zone is separated fromthe vertical skein by a generally horizontal perforated baffle. Theperforations in the baffle determine the size of the relatively verylarge or “coarse” bubbles in a size range effective to scrub outersurfaces of said fibers, preferably from about 10 mm to 50 mm, measuredwithin a coarse-bubble discharging zone about 10 cm from the baffle'sgas-discharging surface. Coarse bubbles leave the trapping zone and exitgenerally parallel to and along the fibers of the vertical skein. Thebaffle is referred to as a “conversion baffle” because it traps andconverts fine bubbles into coarse. Surprisingly, the difficulty ofmaintaining operation of a fine-bubble generating means in a bioreactorhas been effectively dealt with.

It is therefore a general object of this invention to provide afiltration device for the separation of a liquid from a biomasscontaining live microorganisms requiring a gas, typically oxygen forgrowth, the device comprising a vertical skein of fibers resting on aconversion baffle disposed within a biomass contacted with fine bubblesof the gas; the conversion baffle has through-openings which allow gastrapped under the baffle to be discharged as coarse bubbles from adischarging zone near the surface of a gas-distribution supply line.

The amount of gas introduced as fine bubbles under a conversion bafflefor the benefit of microorganisms in the substrate, is preferably atleast enough to generate sufficient coarse bubbles to scrub the fibersand inhibit a build-up of microbes growing on the fibers' surfaces. Anexcellent flow of permeate is maintained over a long period.

A novel composite header is provided for a bundle of hollow fibermembranes or “fibers”, the composite header comprising a molded body ofarbitrary shape, having an upper lamina formed from a liquid “fixing”(potting) material which is formed on a lower lamina of a finelydivided, pulverulent, solid “fugitive” potting material. The terminalportions of the fibers are first potted in the solid fugitive powderymaterial which is removed after the composite header is formed. Ifdesired, the composite header may include additional laminae, forexample, a “cushioning” lamina overlying the fixing lamina, to cushioneach fiber around its embedded outer circumference; and, a “gasketing”lamina to provide a suitable gasketing material against which a permeatecollection means may be sealed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional objects and advantages of the inventionwill best be understood by reference to the following detaileddescription, accompanied by schematic illustrations of preferredembodiments of the invention, in which illustrations, like referencenumerals refer to like elements, and in which:

FIG. 1 is a perspective view of a single array, schematicallyillustrated, of a row of substantially coplanarly disposed parallelfibers secured near their opposed terminal ends between spaced apartcards. Typically, multiple arrays are assembled before being potted.

FIG. 2 illustrates a stack of arrays in end view, clamped together,showing that the individual fibers (only the last fiber of each lineararray is visible, the remaining fibers in the array being directlybehind the last fiber) of each array are separated by the thickness of astrip with adhesive on it, as the stack is held vertically in pottingmaterial.

FIG. 3 is a perspective view illustrating a single skein with itsintegral finished header and permeate collection pan supported on aconversion baffle held between opposed skirts of a shroud.

FIG. 4 is a cross-sectional view of the single skein shown in FIG. 3.

FIG. 5 is a cross-sectional elevational view schematically illustratingmultiple skeins, each with an integral finished header and permeatecollection pan disposed above a conversion baffle held between opposedskirts of a shroud.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the novel method described hereinbelow, the end portions ofindividual fibers are potted in spaced-apart relationship in a finishedheader of cured resin, most preferably by “potting” the end portions offibers sequentially in at least two steps. First, a “fugitive lamina”(so termed because it is removable) is formed with a mass of discretesolid particles. Then, a liquid potting material (referred to as “fixingmaterial”) is solidified or cured after it is deposited upon thefugitive lamina. Upon removing the fugitive lamina, what is left is the“finished” or “final” header formed by the second potting material. Ofcourse, less preferably, any prior art method may be used for formingfinished headers in which opposed terminal end portions of a fibers in astack of arrays are secured in proximately spaced-apart relationshipwith each other.

Permeate may be efficiently withdrawn from a substrate for asurprisingly long period, in a single stage, essentially continuousfiltration process, by mounting a pair of headers in vertically spacedapart relationship, one above another, within the substrate whichdirectly contacts a multiplicity of long vertical fibers in a“gas-scrubbed assembly” comprising a skein, a conversion baffle and agas-distribution means for generating fine bubbles. The skein has asurface area which is at least >1 m², and opposed spaced-apart ends ofthe fibers are secured in spaced-apart headers, so that the fibers, whendeployed in the substrate, acquire a generally vertical profiletherewithin and sway within the bubble zone defined by at least onecolumn of bubbles. The length of fibers between opposed surfaces ofheaders from which they extend, is preferably in the range from at least0.1% (per cent) longer than the distance separating those opposed faces,but less than 5% longer.

Typically permeate is withdrawn from the open ends of fibers whichprotrude from the permeate-discharging aft (upper) face of a header. Theoverall geometry of potted fibers is determined by a ‘fiber-settingform’ used to set individual fibers in an array. The skein operates in asubstrate held in a reservoir at a pressure in the range for MF, undervacuum of about 6 kPa (1 psi abs) up to 1 atm, and for UF from 1 atm toan elevated pressure up to about 10 atm in a pressurized vessel,optionally in addition with vacuum, without the skein being confinedwithin the shell of a module.

Skein fibers are maintained sufficiently free from particulate depositswith surprisingly little cleansing gas, so that the specific flux atequilibrium is maintained over a long period, typically from 50 hr to1500 hr, because the skein is immersed so as to present a generallyvertical profile, and, the skein is maintained awash in bubbles eithercontinuously or intermittently generated by openings in the conversionbaffle. Each header is preferably in the shape of a rectangularparallelpiped, the upper and lower headers having the same transverse(y-axis) dimension, so that plural headers are longitudinally stackable(along the x-axis). A bank of skeins is “gas-scrubbed” with pluralopenings between adjacent skeins, so that for “n” rectangular headersthere are “n+1” rows of openings.

In a typical gas-scrubbed assembly of fibers for liquid filtration, theassembly comprises, (a) bank of gas-scrubbed skeins of fibers whichseparate a desired permeate from a large body of multicomponentsubstrate having finely divided particulate matter in the range from 0.1μm-44 μm dispersed therein, (b) each skein comprising at least 20 fibershaving upper and lower terminal portions potted spaced-apart from eachother, in upper and lower headers, respectively, the fibers beingswayable in a bubble zone, (c) a conversion baffle beneath the skeins,and (d) a gas-distribution means adapted to provide a profusion ofvertically ascending fine bubbles under the conversion baffle in anamount at least sufficient to provide the needs of the microorganisms.The gas-distribution means also provides gas at a flow rate which isproportional to the number of fibers. The flow rate is generally in therange from 0.47-14 cm³/sec per fiber (0.001-0.03 scfm/fiber) (standardft³ per minute per fiber), typically in the range from 1.4-4.2cm³/sec/fiber (0.003-0.009 scfm/fiber). The surface area of the fibersis not used to define the amount of air used because the air travelssubstantially vertically along the length of each fiber. Fine airbubbles preferably have an average diameter in the range from about 1 mmto about 3 mm, and coarse bubbles have an average diameter in the rangefrom about to 10 mm to about 25 mm.

The number of fibers in an array is arbitrary, typically being in therange from about 1000 to about 10000 for commercial applications, andthe preferred surface area for a skein is in the range from 10 m² to 100m². It is most preferred to use a skein made with from 500 to 5000fibers in the range from 1 m to 3 m long, in combination with a permeatecollection means. Typical applications for the system are inmicrofiltration processes used to remove large organic molecules,emulsified organic liquids and colloidal or suspended solids, usuallyfrom water, as for example in a membrane bioreactor, to produce permeateas purified water and recycle biomass. In those instances where a pumpis conveniently used, a vacuum pump is unnecessary, adequate drivingforce being provided by a simple centrifugal pump incapable of inducinga high vacuum, for example of 75 cm Hg on the suction side.

The fibers used to form the skein may be formed of any conventionalmembrane material provided the fibers are flexible and have an averagepore cross sectional diameter for microfilitration, namely in the rangefrom about 1000 Å to 10000 Å. Preferred fibers operate with atransmembrane pressure differential in the range from 6.9 kPa (1 psi)-69kPa (10 psi) and are used under ambient pressure with the permeatewithdrawn under gravity. The fibers are chosen with a view to performtheir desired function, and the dimensions of the skein are determinedby the geometry of the headers and length of the fibers. It isunnecessary to confine a skein in a modular shell, and a skein is not.

Preferred fibers are made of organic polymers and ceramics, whetherisotropic, or anisotropic, with a thin layer or “skin” on the outsidesurface of the fibers. Some fibers may be made from braided cotton orpolymeric fiber covered with a porous natural rubber latex or awater-insoluble cellulosic polymeric material. Preferred organicpolymers for fibers are polysulfones, poly(styrenes), includingstyrene-containing copolymers such as acrylonitrile-styrene,butadiene-styrene and styrene-vinylbenzylhalide copolymers,polycarbonates, cellulosic polymers, polypropylene, poly(vinylchloride), poly(ethylene terephthalate), and the like disclosed in U.S.Pat. No. 4,230,463 the disclosure of which is incorporated by referencethereto as if fully set forth herein. Preferred ceramic fibers are madefrom alumina, by E.I. duPont deNemours Co. and disclosed in U.S. Pat.No. 4,069,157.

For hollow fiber membranes, the outside diameter of a fiber is at least20 μm and may be as large as about 3 mm, typically being in the rangefrom about 0.1 mm to 2 mm. The larger the outside diameter the lessdesirable the ratio of surface area per unit volume of fiber. The wallthickness of a fiber is at least 5 μm and may be as much as 1.2 mm,typically being in the range from about 15% to about 60% of the outsidediameter of the fiber, most preferably from 0.5 mm to 1.2 mm.

The fixing material to fix the fibers in a finished header is mostpreferably either a thermosetting or thermoplastic synthetic resinousmaterial, optionally reinforced with glass fibers, boron or graphitefibers and the like. Thermoplastic materials may be crystalline, such aspolyolefins, polyamides (nylon), polycarbonates and the like,semi-crystalline such as polyetherether ketone (PEEK), or substantiallyamorphous, such as poly(vinyl chloride) (PVC), polyurethane and thelike. Thermosetting resins commonly include polyesters, polyacetals,polyethers, cast acrylates, thermosetting polyurethanes and epoxyresins. Most preferred as a “fixing” material (so termed because itfixes the locations of the fibers relative to each other) is one whichwhen cured is substantially rigid in a thickness of about 2 cm, andreferred to generically as a “plastic” because of its hardness. Such aplastic has a hardness in the range from about Shore D 50 to Rockwell R110 and is selected from the group consisting of epoxy resins,phenolics, acrylics, polycarbonate, nylon, polystyrene, polypropyleneand ultra-high molecular weight polyethylene (UHMW PE). Polyurethanesuch as is commercially available under the brand names Adiprene® fromUniroyal Chemical Company and Airthane® from Air Products. andcommercially available epoxy resins such as Epon 828 are excellentfixing materials.

The particular method of securing the fibers in each of the headers isnot narrowly critical, the choice depending upon the materials of theheader and the fiber, and the cost of using a method other than potting.However, it is essential that each of the fibers be secured influid-tight relationship within each header to avoid contamination ofpermeate. This is effected by potting the fibers essentially vertically,in closely-spaced relationship, either linearly in plural equally spacedapart rows across the face of a header in the x-y plane; oralternatively, randomly, in non-linear plural rows. In the latter, thefibers are displaced relative to one another in the lateral direction.Yet another choice is a cylindrical configuration as disclosed in Ser.No. 08/690,045.

It is most preferred to form a composite header in the form of arectangular prism, which header comprises a “fugitive lamina” offugitive powder; and a “fixing lamina” formed of non-fugitive fixingliquid. By a “fugitive powder” we refer to a material which is in a sizerange so small as to form a packed bed which is essentially impermeableto the fixing liquid under ambient atmospheric conditions. The packedbed may be removed after the composite header is formed, by one ofseveral ways depending upon the powder used. If the powder is silica,pumice, or other fluidizable material, it may be shaken or sucked out(under vacuum) of the mold without dissolving the powder. Preferably thepowder is either (i) soluble in a medium in which the fibers and fixingmaterial are not soluble, or (ii) fluidizable by virtue of having amelting point (if the material is crystalline) below that which mightdamage the fibers or fixing material; or, the powder has a glasstransition temperature Tg (if the material is non-crystalline), belowthat which might damage the fibers or material(s) forming thenon-fugitive lamina; or (iii) both soluble and fluidizable without beingdissolved. Preferred water-soluble powders are common salt (NaCl) orcane or beet sugar; a preferred low melting powder is a low melting waxmelting below 80° C.

The powder is poured around terminal portions of fibers forming a bedembedding the fibers at a depth from about 1 cm to about 5 cm in thefugitive lamina and plugging the ends of the fibers. The fibers in thefugitive lamina are then again potted, this time by pouring the fixingliquid over the fugitive lamina.

In greater detail, the method for forming a finished header comprises,forming a stack of at least two superimposed essentially coplanar andsimilar arrays, each array comprising a chosen number of fiberssupported on a support means having a thickness corresponding to adesired lateral spacing between adjacent arrays;

holding the stack in a fugitive powder to embed terminal portions of thefibers in the powder, forming a fugitive lamina, provided that thepowder is unreactive with material of the fibers; and,

pouring a fixing liquid over the fugitive lamina to embed the fibers toa desired depth, and solidifying the fixing liquid to form a fixinglamina upon the fugitive lamina, the fixing liquid also beingsubstantially unreactive with either the material of the fibers or thatof the fugitive lamina;

whereby a composite header is formed in which terminal portions of thefibers are potted, preferably in a geometrically regular pattern, thecomposite header comprising a laminate of a fugitive lamina of fugitivepowder and a contiguous finished header of fixing lamina; andthereafter,

removing the fugitive lamina without removing a portion of the fixinglamina so as to leave the ends of the fibers open and protruding fromthe aft face of the header, the open ends having a generally circularcross-section.

The step-wise procedure for forming an array with the novel header isdescribed with respect to an array illustrated in FIG. 1, as follows:

A desired number of fibers 12 are each cut to about the same length witha sharp blade so as to leave both opposed ends of each fiber with anessentially circular cross-section. The fibers are coplanarly disposedside-by-side in a linear array on a planar support means such as stripsor cards 15 and 16. Preferably the strips are coated with an adhesive,e.g. a commercially available polyethylene hot-melt adhesive, so thatthe fibers are glued to the strips and opposed terminal portions 12″respectively of the fibers, extend beyond the strips. Intermediateportions 12′ of the fibers are thus secured on the strips.Alternatively, the strips may be grooved with parallel spaced-apartgrooves which snugly accommodate the fibers. The strips may be flexibleor rigid. If flexible, strips with fibers adhered thereto, are in turn,also adhered to each other successively so as to form a progressivelystiffer stack for a header having a desired geometry of potted fibers.To avoid gluing the strips, a regular pattern of linear rows may beobtained by securing multiple arrays on rigid strips in a stack, withrubber bands 18 or other clamping means. The terminal portions 12″ arethus held in spaced-apart relationship, with the center to centerdistance of adjacent fibers preferably in the range from 1.2 (1.2 d) toabout 5 times (5 d) the outside diameter ‘d’ of a fiber. Spacing thefibers further apart wastes space and spacing them closer increases therisk of fiber-to-fiber contact near the terminal end portions when theends are potted. Preferred center-to-center spacing is from about 1.5 dto 2 d. The thickness of a strip and/or adhesive is sufficient to ensurethat the fibers are kept spaced apart. Preferably, the thickness isabout the same as, or relatively smaller than the outside diameter of afiber, preferably from about 0.5 d to 1 d thick, which becomes thespacing between adjacent outside surfaces of fibers in successive lineararrays.

Having formed a first array, a second array (not shown because it wouldappear essentially identical to the first) is prepared in a manneranalogous to the first; strip 15 of the second array is overlaid uponthe intermediate portions 12′ on strip 15 of the first array, the strip15 of the second array resting on the upper surfaces of the fiberssecured in strip 15 of the first array. Similarly, strip 16 of thesecond array is overlaid upon the intermediate portions 12′ on strip 16of the first array.

A third array (essentially identical to the first and second) isprepared in a manner analogous to the first, and then overlaid upon thesecond, with the strips of the third array resting on the upper surfacesof the fibers of the second array.

Additional arrays are overlaid until the desired number of arrays arestacked in rows forming a set of arrays with the adhesive-coated stripsforming the spacing means between successive rows of fibers. The set ofarrays is then held vertically to present one stack of strips (the lowerstack) to be potted first.

Referring to FIG. 2, there is schematically illustrated a rectangularpotting pan 17 the length and width dimensions of which correspondsubstantially to the longitudinal (x-axis) and transverse (y-axis)dimensions respectively, of the desired header. The lower stack issubmerged in a bed of powdered salt having a level indicated by L1, inthe pan 17. Most preferred is a liquid wax, preferably a water-solublewax having a melting point lower than 75° C., such as a polyethyleneglycol (PEG) wax.

The depth of the bed of powder will depend upon whether the strips 15are to be removed from, or left in the finished header.

A. First illustrated is the potting of skein fibers in a upper and lowerheaders from which the strips will be removed.

(1) A fugitive lamina having a thickness L1 from about 5-10 cm thick(corresponding to the depth of the bed) is formed. The depth of the bedis sufficient to ensure that both the intermediate portions 12′ on thestrips and terminal portions 12″ will be embedded and held spaced apart.

(2) The fixing liquid, a curable, water-insoluble potting resin, orreactive components thereof, having a viscosity high enough so as not topenetrate the bed of powder sufficiently to plug the ends of the fibers,is poured over the surface of the fugitive lamina to surround thefibers, until the fixing liquid rises to a level L2. It is solidified toform the fixing lamina 11 (which will be the finished header) having athickness measured from the level L1 to the level L2 (the thickness iswritten “L1-L2”). The thickness L1-L2 of the fixing lamina, typicallyfrom about 1 cm to about 5 cm, is sufficient to maintain the relativepositions of the vertical fibers. A first composite header is thusformed having the combined thicknesses of the fugitive and fixinglaminae.

(3) In a manner analogous to that described immediately hereinabove, astack is potted in a second composite header.

(4) The composite headers are demolded from their potting pans and afterthe salt is shaken out, remaining salt is washed out leaving only thefinished headers, each having a thickness L1-L2. The fugitive powdersuch as salt, may be recovered to be reused.

(5) The adhered strips and terminal portions of the fibers which wereembedded within the fugitive lamina are left protruding from thepermeate-discharging aft faces of the headers with the ends of thefibers being not only open, but essentially circular in cross section.The fibers may now be cut above the strips to discard them as well asthe terminal portions of the fibers adhered to them, yet maintaining thecircular open ends. The packing density of fibers, that is, the numberof fibers per unit area of header preferably ranges from 4 to 50fibers/cm² depending upon the diameters of the fibers.

B. Illustrated second is the potting of skein fibers in upper and lowerheaders from which the strips will not be removed, to avoid the step ofcutting the fibers.

(1) The bed of powder is formed having a level L1′ below the cards, to adepth in the range from about 1-2.5 cm, forming fugitive lamina L1′.

(2) The fixing liquid is then poured over the fugitive lamina to depthL2 and solidified, forming a composite header with a fixing laminahaving a thickness L1′-L2.

(3) The fugitive lamina is removed and the composite header is demoldedleaving the terminal portions 12″ protruding from the aft face of thefinished header, which aft face is formed at what had been the levelL1′. The finished header having a thickness L1′-L2 embeds the strips 15(along with the rubber bands 18, if used).

C. Illustrated third is the potting of skein fibers to form a finishedheaders with a cushioning lamina embedding the fibers on the opposed(fore) faces of the headers from which the strips will be removed.

The motion of fibers being scrubbed generates some intermittent‘snapping’ motion of the fibers. This motion has been found to break thepotted fibers around their circumferences, at the interface of the foreface and substrate. The hardness of the fixing material which forms a“fixing lamina” was found to initiate excessive shearing forces at thecircumference of the fiber. The deleterious effects of such forces isminimized by providing a cushioning lamina of material softer than thefixing lamina. Such a cushioning lamina is formed integrally with thefixing lamina, by pouring cushioning liquid (so termed for its functionwhen cured) over the fixing lamina to a depth L3 as shown in FIG. 2,which depth is sufficient to provide enough ‘give’ around thecircumferences of the fibers to minimize the risk of shearing. Suchcushioning liquid, when cured is rubbery, having a hardness in the rangefrom about Shore A 30 to Shore D 45, and is preferably a polyurethane orsilicone or other rubbery material which will adhere to the fixinglamina. Upon removal of the fugitive lamina, the finished header thusformed has the combined thicknesses of the fixing lamina and thecushioning lamina, namely L1-L3 when the strips 15 are cut away.

D. Illustrated fourth is the formation of a finished header with agasketing lamina embedding the fibers on the header's aft face, and acushioning lamina embedding the fibers on the header's fore face; thestrips are to be removed.

Whichever finished header is made, it is preferably fitted into apermeate pan 20 as illustrated in FIG. 3 with a peripheral gasket. Ithas been found that it is easier to seal the pan against a gasketinglamina, than against a peripheral narrow gasket. A relatively softgasketing material having a hardness in the range from Shore A 40 toShore D 45, is desirable to form a gasketing lamina integrally with theaft face of the finished header. In the embodiment in which the stripsare cut away, the fugitive lamina is formed as before, and a gasketingliquid (so termed because it forms the gasket when cured) is poured overthe surface of the fugitive lamina to a depth L4. The gasketing liquidwhich has a viscosity high enough so as not to penetrate the bed ofpowder sufficiently to plug the ends of the fibers, is then cured. Uponremoval of the fugitive lamina, when the strips 15 are cut away, thefinished header thus formed has the combined thicknesses of thegasketing lamina (L1-L4), the fixing lamina (L4-L2) and the cushioninglamina (L2-L3), namely an overall L1-L3.

Referring to FIG. 3, there is shown a skein 10 held between lower andupper headers 11 and 12, having lower and upper permeate collection pans21 and 22, respectively, sized to snugly accommodate each header 11. Thepan is conveniently formed as a rectangular box having a base ofsubstantially the same length and width dimensions as the header towhich it is to be fitted. The pan 21 rests on a conversion baffle 40having perforations 41 along the entire length on each side of the panso that they extend over a major portion of the perimeter of the header11; and, a V-shaped trough 46 (see FIG. 4) intermediate the oppositelydisposed perforations 41. Preferably the pan 21 is removably secured tothe baffle 40 with fastening means. The baffle 40 is held above thefloor 50 by opposing sidewalls (or skirts) and preferably the baffle isformed integrally with a box-shaped shroud having opposed sidewalls 42and 42′ and opposed end walls 43, 43′ (not visible) to confine the finebubbles beneath the lower header. The sidewalls are high enough,typically 1 to 2 meters, to allow time for the oxygen in the air todissolve in the water, and the sidewalls may have one or more openings44 to facilitate circulation of substrate around the skein fibers 12.

Referring further to the elevational cross-section view in FIG. 4, it isseen that within the sidewalls 42, 42′ and end walls 43, 43′, under thebaffle 40 there is inserted an air supply pipe 60 resting on the floor,the pipe fitted with fine-bubble generators 61. The pan 21 rests abovethe V-shaped trough 46; the zone between the upper surfaces of thefine-bubble generators 61 and a phantom line indicated by referencenumeral 65 is referred to as the fine-bubble discharging zone 66 withinwhich the average diameter of the fine bubbles is measured. Upontravelling upwards, the fine bubbles are trapped as a relatively largemass of air in a trapping zone 48 directly below the baffle 40 and abovethe surface of the substrate, from which trapping zone the air exitsthrough perforations 41 on either side of the header 11 upwardly alongthe skein fibers. The zone between the upper surface of the baffle 40and a phantom line indicated by reference numeral 67 is referred to as acoarse-bubble discharging zone 68 within which the average diameter ofthe coarse bubbles is measured.

Permeate flows from the open ends of the fibers into the pans 21, 22through permeate withdrawal conduits 31 and 32 which may be positionedin the sides of the pans as illustrated, in open flow communication withthe inner portion of the pans. Whether operating under gravity alone, orwith a pump to provide additional suction, it will be apparent that afluid-tight seal is necessary between the peripheries of the headers 11,12 and the upper portions of the pans 21, 22. Such a seal is obtained byusing any conventional means such as a suitable sealing gasket orsealing compound, typically a polyurethane or silicone resin, betweenthe lower peripheries of the headers 11 and the peripheries of the pans.When the skein is backwashed, backwashing fluid flows through the fibersand into the substrate.

It will now be evident that a header with a circular periphery may beconstructed, if desired as described in Ser. No. 08/690,045. Headerswith geometries having still other peripheries (for example, an ellipse)may be constructed in an analogous manner, if desired, but rectangularheaders are most preferred for ease of construction with multiple lineararrays.

In general, the permeate is preferably withdrawn from both the upper andlower headers simultaneously, but may be removed from only the upper orlower header. The skein is operated until the flux declines to so low alevel as to require that the fibers be backwashed. The skeins may bebackwashed by introducing a backwashing fluid through the upper permeatecollection manifold and removing the fluid through the lower manifold,or vice versa. Typically, from 3 to 30 skeins may be coupled togetherfor internal fluid communication with one and another through theheaders, permeate withdrawal means and the fibers; all the skeins may bescrubbed concurrently using a common conversion baffle. Since thepermeate withdrawal means is also used for backflushing it is generallyreferred to as a ‘liquid circulation means’, and as a permeatewithdrawal means only when it is used to withdraw permeate.

Referring to FIG. 5 there is schematically illustrated a sideelevational view of 10 skeins 10 with the lower header of eachpositioned over a V-shaped trough in a baffle 70. Multiple fine-bubblegenerators 61 are positioned in longitudinal rows between sidewalls 72,72′ of the shroud indicated generally by 75.

The type of gas (air) manifold is not narrowly critical provided itdelivers fine bubbles in the amount necessary for the purpose at hand.

The air may be provided continuously or intermittently, better resultsgenerally being obtained with continuous air flow. The amount of airprovided depends upon the type of substrate, the requirements of thetype of microorganisms, if any, and the susceptibility of the surfacesof the fibers to be plugged, there always being sufficient air toproduce desired growth of the microorganisms when operated in asubstrate where maintaining such growth is essential.

EXAMPLE 1

An aerobic biochemcial treatment system has a daily capacity of 10,000cubic meters of sewage with and oxygen demand of 500 mg/L representing atotal oxygen demand of 5000 Kg/day. The bioreactor is a rectangular tank25 meters long, 20 meters wide with a working depth of 6 meters.Microfiltration is carried out with 300 skeins, each 2 meters high andhaving 50 m² of surface. Each skein can process 1400 liters/hr using 30cubic meters/hr of scouring air. All measurements of volume of air aregiven at standard conditions of 1 atm. and 20° C. at sea level.

The skeins are submerged so that their lower headers are about 2 metersbelow the surface of the liquid in the bioreactor. The aerators arepositioned about 4 meters below the lower header.

The amount of air required to scour (or “air-scrub”) all skeins is216,000 m³/day and this is most effectively accomplished with coarsebubbles.

The transfer efficiency in the bioreactor with coarse bubble aerators is1% per meter of the depth below the skein that the coarse bubble aeratoris positioned. The transfer efficiency with fine bubble aerators is 3%per meter of the depth below the skein that the fine bubble aerator ispositioned.

In addition to the air required for scouring, the oxygen demand of themicrobes must be supplied with additional air, referred to as “auxiliaryair”. In a bioreactor fitted with vertical skeins and no conversionbaffle, so that all air is supplied as coarse bubbles, the auxiliary airrequired is 3 times greater than the auxiliary air supplied as finebubbles (which transfer available oxygen 3 times more efficiently thanthe coarse bubbles at the same depth). Coarse bubbles are supplied withAeroflow® stainless steel coarse bubble diffuser, from AeroflowEnvironmental Inc. Fine bubbles are supplied with Sanitaire® flexiblemembrane tube diffusers available from Sannitaire Water PollutionControl Corp. and Elastox® non-clog fine bubble rubber diffusers fromEimco Process Equipment Co. The trapping zone under the conversionbaffle is estimated to be narrow, in the range from about 1 cm to about5 cm in height.

The air requirements for the bioreactor using a conversion baffle andfor the same bioreactor using coarse bubbles only, are set forth below,side by side. All volumes of air are set forth in the following Table 1,as standard cubic meters per day.

TABLE 1 With Conv. Baffle Coarse bubbles only Air reqd. to scour allskeins 216,000 216,000 Air reqd. to provide oxygen demand  68,000204,000 Total air reqd. 284,000 420,000

It is evident that using a conversion baffle yields a saving of nearly50% in the cost of supplying air.

In each case permeate is withdrawn through lines connected to thecollection pan of each header with a pump generating about 34.5 kPa (5psi) suction. Permeate is withdrawn at a specific flux of about 0.7lm²h/kPa yielding about 4.8 l/min of permeate which has an averageturbidity of <0.8 NTU, which is a turbidity not discernible to the nakedeye.

Having thus provided a general discussion, and specific illustrations ofthe best mode of constructing and deploying a membrane device with aconversion baffle, and having provided specific illustrations in whichusing the conversion baffle is compared with not using it, it is to beunderstood that no undue restrictions are to be imposed by reason of thespecific embodiments illustrated and discussed, and particularly thatthe invention is not restricted to a slavish adherence to the detailsset forth herein.

What is claimed is:
 1. A header in which a multiplicity of hollow fibermembranes or “fibers” is potted, said header comprising, a molded bodyof arbitrary shape striated in a fixing lamina in an upper portion and afugitive lamina in a lower portion, said fugitive lamina formed from afugitive powdery material and said fixing lamina formed from a fixingliquid integrally with a permeate collection pan; said fibers havingterminal portions thereof potted in said fugitive lamina which as apacked bed plugs ends of said fibers, said ends having an essentiallycircular cross-section, said fugitive lamina maintaining said ends inclosely spaced-apart substantially parallel relationship; said fugitivelamina having an aft face through which said ends protrude, and a foreface through which said fibers extend vertically; said fugitive laminahaving said fixing lamina adhered thereto, said fixing lamina having athickness sufficient to maintain said fibers in substantially the samespaced-apart relationship relative to one and another as thespaced-apart relationship in said lower portion.
 2. The header of claim1 wherein said fixing lamina has a cushioning lamina embedding saidfibers and coextensively adhered to said fixing lamina, said fixinglamina has a hardness in the range from about Shore D 50 to Rockwell R110, and said cushioning layer has a hardness in the range from Shore A30 to Shore D 45.